1. Field of the Invention
The present invention relates to a display device having a liquid crystal display, for example a head-up display, and to a method for protecting the liquid crystal display of a corresponding display device against incident solar radiation.
2. Related Art
Display devices having liquid crystal displays should not be exposed to direct and intense optical radiation, such as solar radiation, for a prolonged period of time. Owing to the radiation that is incident on the liquid crystal display, the temperature of the liquid crystal display increases and the display may lose its physical properties if the temperature increases too much. In this case, the display clears, and any displayed image information is lost. Even though such a process is generally reversible, irreversible damage may occur if the liquid crystal display is exposed for a prolonged period of time or relatively frequently.
In motor vehicles having a head-up display (HUD) as a display device, solar rays can pass through a windscreen of the vehicle and, in the case of an unfavorable position of the Sun, impinge directly on the display. The head-up display or front-view display projects an image through a beam path onto the windscreen of the vehicle, which serves as the projection surface, which image can be perceived by the occupants of the vehicle as a virtual image. However, as long as the optical radiation is incident at a particular angle, not only the image is projected, but also the radiation that is incident on the vehicle is guided directly onto the display and concentrated on the display surface owing to a mirror optical system present in the beam path of the head-up display. If the liquid crystal display is irreversibly damaged, for example when stopping in the sunshine for a long period of time, what is referred to as a “bonanza effect” can occur in a virtual image displayed by the head-up display, in which artifacts caused by the irreversible damage remain visible in the virtual image.
In order to avoid such irreversible damage to a liquid crystal display, in the case of the head-up displays known from the prior art, for example a polarization filter is mounted in front of the display, which lets only a portion of the incident radiation having a particular polarization direction through, with this polarization direction corresponding to a polarization direction of the display. By way of example, only s-polarized light may pass through the polarization filter. As a result, heating of the liquid crystal display is reduced and the point of time at which the bonanza effect first occurs is delayed. The disadvantage here is that the polarization filter decreases the performance and image sharpness of the liquid crystal monitor and that increased power consumption and associated heat losses are necessary in order to achieve a sufficiently high transmission of the liquid crystal display despite the filters.